Method And Installation For Enriching A Gas Stream With One Of The Components Thereof

ABSTRACT

The invention relates to a method of enriching a pressurised gas stream ( 1 ) with one of the components (A) thereof. The inventive method comprises the following steps: the stream is separated into at least first and second fractions ( 2, 3 ); at least one part of the first fraction ( 2 ) is sent to a separation unit (ASU); the separation unit supplies at least two discharges, including a first discharge ( 10 ) having a greater A content than that of the fraction ( 2 ) supplied to the separation unit; at least one part of the first discharge ( 10 ) is mixed with at least one part of the second fraction ( 3 ) such as to form a pressurised gas mixture ( 15 ); the second fraction ( 3 ) is expanded and, subsequently, at least one part of the first discharge ( 10 ) is mixed therein.

This application is a division of U.S. application Ser. No. 10/577,621which was filed on Feb. 21, 2007 which is a §371 of International PCTApplication PCT/FR2004/050570, filed Nov. 5, 2004, the entire contentsof which are incorporated herein by reference.

The present invention relates to a method and to an installation forenriching a gas stream with one of its constituents. In particular, itrelates to a method of enriching air with oxygen.

The oxygen enrichment of air has become necessary in the iron and steelindustry.

The reduction or elimination of hot coke in blast furnaces, generally tothe benefit of coal powder injection (CPI), requires this necessarychange.

The means known from EP-A-0 531 182 for economically achieving thisenrichment consists in cryogenically distilling one portion of thestream of air for the blast furnace. What is thus obtained is anitrogen-rich stream and an oxygen-rich stream, the latter then beingremixed into the stream of air downstream of the air separation unit.

Since the pressure of the oxygen stream is close to that of the airstream feeding the air separation unit (ASU), a method involving amixing column will prove to be particularly suitable and economic.

FIG. 1 shows a separation unit described in EP-A-0 531 182 intended forenriching air with oxygen. It is fed from the air system constitutingthe charge for a blast furnace at a pressure P. The air distillationunit is intended to produce low-purity oxygen, for example having apurity of 80 to 97% and preferably 85 to 95%, at a defined pressureslightly above the pressure P, for example advantageously at a pressureof 1×10⁴ Pa abs to 1×10⁵ Pa above the pressure P.

The unit essentially comprises a heat exchange line 1A, a doubledistillation column 2A, which itself comprises a medium-pressure column3A, a low-pressure column 4A and a main condenser-reboiler 5A, and amixing column 6A. The columns 3A and 4A typically operate at about5.45×10⁵ Pa and about 1.5×10⁵ Pa respectively.

As explained in detail in document U.S. Pat. No. 4,022,030, a mixingcolumn is a column having the same structure as a distillation columnbut used for mixing in a manner close to reversibility, a relativelyvolatile gas, introduced at the bottom of the column, with aless-volatile liquid, introduced at the top of the column.

Such mixing generates refrigeration energy and therefore allows theenergy consumption associated with the distillation to be reduced. Inthe present case, this mixing is also profitably used for impure oxygento be produced directly at the pressure P, as will be described below.

In the case of FIG. 1, an airstream is compressed to the pressure of themixing column by a compressor 14A, cooled in the exchange line 1A,subcooled in the subcooler 21A and sent to the bottom of the mixingcolumn 6A.

“Rich liquid” (oxygen-enriched air), withdrawn from the bottom of thecolumn 3A, is, after being expanded in an expansion valve 10A,introduced into the column 4A. “Lean liquid” (impure nitrogen) withdrawnfrom an intermediate point 11A on the column 3A is, after being expandedin an expansion valve 12A, introduced into the top of the column 4A,constituting the waste gas of the installation, which gas and the puregaseous nitrogen at medium-pressure possibly produced at the top of thecolumn 3A are warmed in the exchange line 1A and discharged from theinstallation. These gases are indicated by NI and NG in FIG. 1,respectively.

Liquid oxygen, of greater or lesser purity depending on the setting ofthe double column 2A, is withdrawn from the bottom of column 4A, raisedby a pump 13A to a pressure P1, slightly above the aforementionedpressure P, in order to take account of the pressure drops (P1-P lessthan 2×10⁵ Pa), and introduced into the top of the column 6.

Three fluid streams are withdrawn from the mixing column 6A: from itsbase, liquid similar to the rich liquid and joined with the latter via aline 15A provided with an expansion valve 15A′; from an intermediatepoint, a mixture essentially consisting of oxygen and nitrogen, which issent to an intermediate point on the low-pressure column 4A via a line16A provided with an expansion valve 17A; and, from its top, impureoxygen which, after being warmed in the heat exchange line, isdischarged, substantially at the pressure P, from the installation via aline 18A as production gas OI.

The figure also shows auxiliary heat exchangers 19A, 20A, 21A forrecovering available refrigeration from the fluids circulating in theinstallation.

FIG. 2 shows schematically an integrated apparatus for enriching anairstream intended for a blast furnace according to the prior art.

An airstream is compressed in a blower S, so as to form a compressedstream 1. This stream is divided into two fractions 2 and 3. The firstfraction 2 is cooled by a chiller R, for example a water chiller,compressed in a booster C and sent to an air separation unit (ASU). Theair separation unit operates for example by cryogenic distillation andincludes, upstream of the separation columns, a purification unit and anexchange line. It produces a stream 10 of oxygen containing between 80and 95 mol % oxygen and a nitrogen stream 11, which may be a wastestream. At least one portion of the oxygen-enriched stream 10 is mixedwith the second air fraction 3. The oxygen-enriched, mixed stream 15 isheated in a cowpers W and sent to a blast furnace BF.

To counteract the pressure drops in the circuit comprising the airseparation unit (between the air intake on the blast furnace wind to theseparation unit and reinjection of the oxygen stream), a compressor Cwill be installed. This makes it possible to raise the pressure of thetotal airstream intended for the air separation unit (according to FIG.2) or (as a variant of FIG. 1) of the airstream intended for feeding themixing column (i.e. about 30% of the stream of air treated by theseparation unit).

It is an object of the invention to integrate an air separation unitinto this steelmaking process in a more economic and more reliablemanner, without the use of any gas stream compressors in the airseparation unit other than those connected to the shaft of the expansionturbine for maintaining the refrigeration of the separation unit.

One subject of the invention is a method of enriching a pressurized gasstream with one of its constituents A, which comprises the steps of:

-   -   i) dividing the stream into at least first and second fractions;    -   ii) sending at least one portion of the first fraction into a        separation unit;    -   iii) supplying, from the separation unit, at least first and        second streams, the first stream of which has a content of        constituent A greater than that of the first fraction;    -   iv) mixing at least one portion of the first stream with at        least one portion of the second fraction in order to form a        pressurized gas mixture, characterized in that the second        fraction is expanded before at least one portion of the first        stream is mixed therewith.

According to other optional aspects:

-   -   the pressurized gas stream and the first fraction are        substantially at the same pressure and, in particular, only the        pressure drops are the cause of a variation in pressure between        these two fluids;    -   the first stream and the expanded second fraction are        substantially at the same pressure and, in particular, only the        pressure drops are the cause of a variation in pressure between        these two fluids;    -   the separation unit is autonomous in terms of energy        requirements for compressing the gas streams produced by the        unit or intended for the unit;    -   the pressurized gas stream is air and optionally constituent A        is oxygen;    -   the pressurized gas stream is air intended for a blast furnace;    -   the separation unit is a cryogenic distillation separation unit;    -   the separation unit comprises a medium-pressure column, a        low-pressure column thermally coupled to the medium-pressure        column, and a mixing column; and    -   no portion of the first fraction intended for a distillation        column is compressed or no portion of the first fraction        intended for the mixing column or for the medium-pressure column        is compressed after the stream is divided.

According to one particular method of operation i) in a first operation,at least one portion of the first fraction is compressed and the secondfraction is not expanded before at least one portion of the first streamis mixed therewith; and

-   -   ii) in a second operation, (for example when the compressor C        isn't working) at least one portion of the first fraction is not        compressed (the first fraction is not compressed) and the second        fraction is expanded before at least one portion of the first        stream is mixed therewith.

Another subject of the invention is an installation for enriching apressurized gas stream with one of its constituents A, which comprises:

-   -   i) means for dividing the pressurized gas stream into at least        first and second fractions;    -   ii) a separation unit;    -   iii) means for sending at least one portion of the first        fraction to the separation unit; and    -   iv) means for mixing at least one portion of a first stream,        produced by the separation unit and enriched in A compared to        the first fraction, with the second fraction in order to form a        stream enriched in A compared to the pressurized gas stream,        characterized in that it includes a means for expanding the        second fraction upstream of the means for mixing at least one        portion of the first stream therewith, and downstream of the        means for dividing the gas stream.

According to other optional aspects:

-   -   the separation unit is an air separation unit comprising a        medium-pressure column, a low-pressure column thermally coupled        to the medium-pressure column, and a mixing column;    -   the installation does not include any means for compressing air        intended for the medium-pressure column or for the mixing        column; and    -   the installation includes means for compressing the second        fraction and means for forwarding the second fraction to be        mixed with at least one portion of the first stream without        passing via the expansion means.

Advantageously, the separation method will use a mixing column operatingat a pressure equal to or higher than the pressure of themedium-pressure column, without the need for an additional aircompression means.

It is thus proposed to integrate a mixing-column unit into a blastfurnace blaster without an additional air compressor, thereforeincreasing the reliability of delivery of oxygen molecules and thereforeof enriched air to the blast furnace, while minimizing the investmentneeded for this construction.

Another subject of the invention is a method of separating air using anapparatus comprising at least one medium-pressure column, a low-pressurecolumn thermally coupled to the low-medium-pressure column, and a mixingcolumn operating at a pressure above the pressure of the medium-pressurecolumn, in which:

-   -   i) air, compressed and purified, is sent to the medium-pressure        column;    -   ii) nitrogen-enriched and oxygen-enriched streams are sent from        the medium-pressure column to the low-pressure column;    -   iii) an oxygen-enriched liquid is sent from the low-pressure        column to the top of the mixing column; and    -   iv) an oxygen-enriched gas is withdrawn from the top of the        mixing column,        characterized in that a nitrogen-enriched liquid stream is        withdrawn from the medium-pressure column, pressurized and at        least partly vaporized, and the bottom of the mixing column is        fed with at least one portion of the vaporized liquid.

Preferably, the nitrogen-enriched liquid is vaporized by heat exchangewith part of the feed air. The air thus liquefied may be sent to atleast one of the medium-pressure and low-pressure columns.

The nitrogen-enriched liquid is pressurized by a pump and/or byhydrostatic pressure.

Another subject of the invention is an air separation installationcomprising:

-   -   a) a medium-pressure column;    -   b) a low-pressure column thermally coupled to the        low-medium-pressure column;    -   c) a mixing column operating at a pressure above the pressure of        the medium-pressure column;    -   d) means for sending compressed purified air to the        medium-pressure column;    -   e) means for sending nitrogen-enriched and oxygen-enriched        streams from the medium-pressure column to the low-pressure        column;    -   f) means for sending an oxygen-enriched liquid from the        low-pressure column to the top of the mixing column; and    -   g) means for withdrawing an oxygen-enriched gas from the top of        the mixing column,        characterized in that it includes means for withdrawing a        nitrogen-enriched liquid stream from the medium-pressure column,        means for pressurizing the liquid, means for at least partly        vaporizing the liquid and means for feeding the bottom of the        mixing column with at least one portion of the vaporized liquid.

The invention will be described in greater detail with reference toFIGS. 3, 4 and 5. FIGS. 3 and 5 show a unit for enriching a gas streamaccording to the invention and FIG. 4 shows a particularly suitableseparation unit for carrying out the invention.

FIG. 3 shows schematically an integrated unit for the enrichment of anairstream intended for a blast furnace according to the prior art.

A stream of air is compressed in a blower S in order to form acompressed stream 1. This stream is divided into two fractions 2 and 3.The first fraction 2 is cooled by means of a chiller R, for example awater chiller, and sent to an air separation unit (ASU) without beingcompressed between the chiller and the inlet of the air separation unit.The air separation unit operates for example by cryogenic distillationand includes a purification unit and an exchange line upstream of theseparation columns. It produces an oxygen stream 10 containing between80 and 95 mol % oxygen and a nitrogen stream 11, which may be a wastestream. The second air fraction 3 is expanded by means of an expansionmeans V, which may for example be a valve, an orifice, areduced-diameter pipe or a turbine. At least one portion of theoxygen-enriched stream 10 is mixed, downstream of the expansion means V,with the expanded second air fraction 3. The oxygen-enriched, mixedstream 15 is heated in a cowpers W and sent to a blast furnace BF.

This solution dispenses with the air booster for raising the pressureupstream of the air separation unit. The consumption of energy of thewhole system will therefore be better.

FIG. 4 adopts elements of FIG. 1 having the same reference numerals,which will not be described in detail.

The purified air 7 a at the medium pressure of 5.45 bara coming from themain air compressor for the blast furnace wind or from an expansionturbine is separated into at least two separate flows before enteringthe medium-pressure column 2A.

The first flow 100 is fed directly into the bottom of themedium-pressure column 2A in gaseous form.

The second flow 200 is at least partly condensed in a heat exchanger101A. The liquefied portion is introduced into one of the distillationcolumns (either the medium-pressure column 2A or the low-pressure column4A). In FIG. 4, the stream 202 is sent to the bottom of themedium-pressure column, whereas the stream 204 is sent to thelow-pressure column after being subcooled in the exchanger 19A.

A liquid stream 300 enriched in nitrogen compared to air is withdrawnfrom the medium-pressure column 3A, compressed by means of a pump 400 orby a simple hydrostatic height, vaporized in the heat exchanger 101Aagainst the condensation of medium-pressure air, in order to form agaseous nitrogen stream 500 which is then fed into the bottom of themixing column 6A. Thus, profiting from the difference in compositionbetween the air and the nitrogen-enriched stream, the feed for themixing column 6A takes place at a pressure above that of the air 100feeding the medium-pressure column 3A, and does so without an additionalcompressor.

It is also conceivable to warm the gaseous nitrogen 500 in the mainexchange line before introducing it into the mixing column.

To produce a gaseous nitrogen stream 500 at 5.9 bara, the heat exchanger101A has a ΔT of 0.6° C.

The stream 15A coming from the bottom of the mixing column 6A, beingricher in nitrogen than that of FIG. 1, is sent to just below the top ofthe low-pressure column 4A.

The subcooler 21A is omitted and there is no longer any withdrawal ofmedium-pressure gaseous nitrogen NG.

Optionally, a third flow of air is sent to a booster 8A, cooled in theexchange line 1A and expanded in the blowing turbine 9A, but other meansof refrigeration are conceivable, including expansion of the airintended for the medium-pressure column.

If this booster is present, the advantage of the invention is that thereis no need for an air compression step for air intended for the mixingcolumn or for the medium-pressure column.

In the case of FIG. 4, the extraction efficiency is reduced and theseparation energy of the system remains superior to the base case.

However, integrating the air separation unit of FIG. 4 into the schemedisclosed in the variant shown in FIG. 3 does allow the pressure drop inthe valve to be considerably reduced.

FIG. 5 shows schematically an integrated unit for enriching a stream ofair intended for a blast furnace according to the prior art.

A stream of air is compressed in a blower S in order to form acompressed stream 1. This stream is divided into two fractions 2 and 3.The first fraction 2 is cooled by means of a chiller R, for example awater chiller, compressed in a booster C and sent to an air separationunit (ASU). The air separation unit operates for example by cryogenicdistillation and includes a purification unit and an exchange lineupstream of the separation columns. It produces an oxygen stream 10containing between 80 and 95 mol % oxygen and a nitrogen stream 11,which may be a waste stream. The second air fraction 3 is expanded bymeans of an expansion means V, which may for example be a valve, anorifice, a reduced-diameter pipe or a turbine. At least one portion ofthe oxygen-enriched stream 10 is mixed, downstream of the expansionmeans V, with the expanded second air fraction 3. The oxygen-enriched,mixed stream 15 is heated in a cowpers W and sent to a blast furnace BF.The booster C and the valve V have short-circuiting means. In a firstoperation of the unit, the first fraction 2 is compressed and the secondfraction is not expanded. In a second operation, at least one portion ofthe first fraction is not compressed and the second fraction is expandedbefore at least one portion of the first stream is mixed therewith.

Evaluation of the Variants: Prior Art

BLOWER Air sent to the ASU O₂ at the BF Enriched air at the BF FLOW RATESm³/h 400 000    146 700    3 748   95% O₂ 284 048    eff. CompositionN₂    0.7811    0.7811   0.03    0.700 O₂    0.2096    0.2096   0.95   0.290 Ar    0.0093    0.0093   0.02    0.010 1 1 1 1 PRESSURE bara  5.85   5.55   5.50   5.50 ENERGY kW 30 686    1201   31 887   VARIANT 1 with an Expansion Valve (FIG. 3)

BLOWER Air sent to the AsU O₂ at the BF Enriched air at the BF FLOW RATESm³/h 400 000    146 700    30 748    95% O₂ 284 048    0 0 0 eff. 0Composition N₂    0.7811    0.7811   0.03     0.700 O₂    0.2096   0.2096   0.95    0.290 Ar    0.0093    0.0093   0.02    0.010 1 1 1 1PRESSURE bara   6.85   6.55   5.50   5.50 ENERGY kW 33 428    33 428   VARIANT 2 with Expansion Valve (FIG. 3) and Air Separation Method ofFIG. 4

BLOWER Air sent to the ASU O₂ at the BF Enriched air at the BF FLOW RATESm³/h 417 259    163 959    30 748.32    85% O₂ 284 048    eff.Composition N₂    0.7811    0.7811 0.03    0.700 O₂    0.2096    0.20960.95    0.290 Ar    0.0093    0.0093 0.02    0.010 1 1 1    1 PRESSUREbara   6.23   5.93 5.50 ENERGY kW 33 151    33 151    Prior art VARIANT1 VARIANT 2 REF. CASE Air blower intended for the mixing column Overallcost 100 89 96 95 kW 100 105 104 90

1-18. (canceled)
 19. A method of enriching a pressurized gas stream withone of its constituents A, which comprises the steps of: a) dividing thestream (1) into at least first and second fractions (2, 3); in whichsaid gas stream (1) is air and constituent A is oxygen. b) sending atleast one portion of the first fraction (2) into a separation unit(ASU); c) supplying, from the separation unit, at least first and secondstreams, the first stream (10) of which has a content of constituent Agreater than that of the first fraction; and d) mixing at least oneportion of the first stream with at least one portion of the secondfraction in order to form a pressurized gas mixture (15), characterizedin that the second fraction is expanded before at least one portion ofthe first stream is mixed therewith.
 20. The method of claim 19, inwhich the pressurized gas stream is air intended for a blast furnace(BF).
 21. The method of claim 19, in which the separation unit is acryogenic distillation separation unit (ASU).
 22. (canceled) 23.(canceled)
 24. The method of claim 19, in which: a) in a firstoperation, at least one portion of the first fraction is compressed andthe second fraction is not expanded before at least one portion of thefirst stream is mixed therewith; and b) in a second operation, at leastone portion of the first fraction is not compressed (the first fractionis not compressed) and the second fraction is expanded before at leastone portion of the first stream is mixed therewith. 25-28. (canceled)